1. Field of the Invention
The present invention generally relates to lithography, and more particularly to systems and methods for particle detection.
2. Related Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles may be changed during the lithographic process.
Lithography systems project mask pattern features that are extremely small. Dust or extraneous particulate matter appearing on the surface of the reticle can adversely affect the resulting product. Any particulate matter that deposits on the reticle before or during a lithographic process is likely to distort features in the pattern being projected onto a substrate. Therefore, the smaller the feature size, the smaller the size of particles that it is critical to eliminate from the reticle.
A pellicle is often used with a reticle. A pellicle is a thin transparent layer that may be stretched over a frame above the surface of a reticle. Pellicles are used to block particles from reaching the patterned side of a reticle surface. Any particles on the pellicle surface are out of the focal plane and should not form an image on the wafer being exposed. However, it is still preferable to keep the pellicle surfaces as particle-free as possible.
In lithography, reticle inspection systems may be integrated with lithography tools. This integration can not only protect against printed defects caused by particles ranging from very small to very large, but can also detect crystallization on the reticle, allowing as-late-as-possible reticle cleaning, which can in turn increase machine throughput and utilization. Condition-based reticle cleaning also enables cleaning frequency reduction, which can extend the life of reticles.
Currently used high throughput lithography tools employ rapid in-situ reticle inspection devices to detect particulate contamination. Requirements of speed and a high signal-to-noise ratio have led to utilization of a probe imaging technique, a type of scatterometry, for this purpose. This technique is based on collecting scattered light from contamination and dust particles that have been illuminated in a reasonably small spot on the reticle surface. A reasonably sized spot (e.g., approximately 50 μm to 300 μm) is rastered, or scanned, over a test surface, collecting information from one spot at a time. For a 150 mm by 120 mm surface, this corresponds to an approximately 9 Mpixel image. The probe beam technique is demonstrated in FIGS. 1A-1C, 2, and 3. FIG. 1A depicts a portion 102 of an object, such as a reticle or pellicle. Probe beam spots 104 are shown, with arrow 106 showing the scan direction of a probe beam. A particle 108 is shown on one of probe beam spots 104.
When using the probe beam technique, detection of particles smaller than 100 μm requires intensity quasi-calibration to represent particle size. The size of a particle is commonly determined in terms of Latex-Sphere Equivalence using sets of 5, 10, 30, and 50 μm latex spheres for intensity calibration. For particles up to 50 μm, interpolation of the scattered light amount may be necessary, and for particles up to 100 μm, extrapolation of the scattered light amount may be necessary. As shown in FIG. 1B, when an assembly of small particles within an illumination spot (e.g., probe beam spot 104) is present, this assembly may be recorded as one particle size equivalent to an effective signal collected within a specified pixel 110. Furthermore, an assembly of unresolved small particles may be reported as a single large particle 112, as shown in FIG. 1C.
FIG. 2 shows an example of the scanning, or rastering, of a surface of an object 220 that occurs when using the probe beam technique. In this example, a probe beam spot 204 is scanned in direction 246 while object 220 moves in direction 248. Because object 220 is scanned one probe beam spot at a time, a full assessment of the surface of object 220 is not complete until the scans are all complete. Furthermore, if a pellicized reticle is being assessed, it becomes necessary to run two complete sequential scans in order to assess both the reticle surface and the pellicle surface.
Using the probe beam technique, reflected scattered light is sampled and converted into a grey level bitmap, which can then be mapped as shown in map 350 in FIG. 3. Map 350 shows reported particles 352 that are greater than a particular size (e.g., greater than 10 μm).
The ultimate spatial resolution of the probe beam technique is defined by the beam spot size and pixel size, determined in its turn by designated collection time multiplied by raster speed. Spatial resolution is approximately equal to the spot autocorrelation function, or approximately to a double beam spot size, which can be improved with elaborate image processing, allowing resolution close to the illumination spot size to be achieved. However, detection optics only collect the scattered light during the time allocated for single pixel exposure, and do not resolve the illumination spot. Despite low spatial resolution, sub-pixel sized bright particles can be detected due to a very good signal-to-noise ratio, but cannot be imaged.
The probe beam technique also requires very elaborate and bulky scanning mechanisms that necessitate low numerical aperture (NA) optics in both illumination and detection paths. Use of low numerical aperture optics in the detection path results in a large depth-of-focus (DoF). This leads to inadvertently imaging real structures and objects formed by optical cross-talk that are far from the intended plane of inspection. As a result, probe-based inspection systems fail to adequately recognize contamination particles and fail to discriminate them from optical cross-talk images. Therefore, precision, quality, and certainty of particle size detection is very often compromised when using the probe beam technique.
Requirements for increased scan speed and stability are now limiting the ability of techniques such as the probe beam technique to be effective. In addition, increased scan speed decreases signal-to-noise ratios, limiting the ability to differentiate small particles from noise. Furthermore, the probe beam technique is susceptible to optical cross-talk as line widths shrink. With increasing demands for higher throughput and shrinking lithographic feature sizes, it is becoming increasingly important to enhance a particle detection system's performance in terms of speed, smaller particle size detection, and immunity against unwanted effects such as optical cross-talk. Given the foregoing, what is needed are systems and methods for particle detection using true imaging.